What Portable Headphone amp/DAC should I buy? Ibbasso D4 or D6. Practical Devices XM6 or Pico...

Hello everybody. OK I'll try to put this as simply as I can and I would appreciate it if all of you high tech gurus that know everything would please put your replies in English. I guess you could say that I need a Portable Headphone amp/DAC for Dummies book. I'm new to 24bit audio I put a Asus Xonar Essence ST sound card in my desktop rig and downloaded some HD Tracks.com sample files and played them back through my old Sennheiser HD540,s that I've had for almost 16 years and was BLOWN AWAY and now I'm addicted to 24bit audio. I plan to amass a collection of 24/96 FLAC files as well as my collection of apple lossless files that I ripped from my CD collection and play from my Ipod and Itunes. I'm looking for a portable headphone amp/DAC for my notebook computers and my Ipod using my Shure SE535 IEM as I cannot accept the sound that the notebook soundcard DAC's puts out after listening to 24bit FLAC from the Xonar essence ST. I'm looking at the Nuforce UDAC2, the Ibbasso D4 or D6. The practical devices XM6 and finally the Pico headphone amp/upsampling DAC. I'm willing to pay for quality, the Pico headphone amp/upsampling DAC seem to be considered the best however I'm looking at all my options. The UDAC2 looks like the most cost effective option at $129 however it does not have a line input for my ipod touch and I want an all in one solution, a USB soundcard, headphone amp and external DAC. Something that I take with me everywhere that I go. USB powered for the notebook computers and battery powered for the Ipod touch. I want a solution that does 24 bit audio well but from what I've been learning Windows 7 will only output 16 bits 48 Khz through the USB port. Please will somebody clarify this fact for me. What Bit depth and sampling rate can I expect to get from the USB port of my notebook computer using Windows 7 with a 24 bit 96khz FLAC source file? If it is only 16 bit 48 Khz sampling rate then what will upsampling do for me? and what kind of sound quality will i get? I'm also looking at the Ibbasso D4 and the D6. Both these 2 USB DAC/ Headphone amps have good specs but I do not understand what is the difference between them. The D4 is $199 and the D6 is $276. What do I get with the D6 for extra $76? And lastly the Practical Devices XM6 and the PICO headphone amp/upsampling DAC. I have to really think about getting the Pico for $499, the XM6 is a little easier to swallow at $399. It seems to broil down to the XM6 vs the D6. The Practical Devices XM6 is $399 and the Ibbasso D6 is $276 so simply put, which one is better?